Why is object deviating from path random?
First you have 2 curves overlaying, curba1 and curba2, so it's a little bit confusing. About the useful one, curba2: it's very long, so you need to increase its resolution if you want the movement of the sphere to be smooth (in the Properties panel > Data > Shape > Resolution > Preview and Render). Also, you've given an animation to your sphere (which makes it move away from the curve) but I guess it's on purpose.